astronomer1

The links will get you to 3 pictures of how I made an adjustable air bleed on my old Tarno carb. so it will run reliably and be safely used on an RC craft. The details are as follows to a disassembled carb.:

1. Original air bleed hole enlarged to appx. twice the original size and de-burred both inside and out.

2. Carefully flatten face of carb. using #400 wet /dry sand paper on a flat glass surface.

3. Drill and tap a hole on flat face of carb. for a 2 x 56 screw. Not a thru hole but appx. 3/16" deep. Requires two tap styles for threading. One a plug tap to start and finished with a bottoming tap. Exact position not measured, but was positioned by eye.

4. A small sliding/rotating "door" was fabricated from a scrap piece of aluminum, appx.3/32"x1/4"x3/8". One end was rounded and drilled for a tight 2x56 clearance hole for a 2x56 screw. The mating face of this little widget was carefully flattened using #400 on a glass surface. This was done to insure a good seal against the carb. face.

5. A 1/4" long 2x56 Pan Head Phillips screw and a lock washer secure the widget to the carb. and allow adjustment by slicing the bleed hole very precisely. This whole setup must have a low profile to avoid being hit by the prop. Unfortunately it can not be adjusted with the engine running. Screw must be tightened well to avoid loosing setting due to vibration.

Result: 7k to 18K at 5800' ASL., Hi-Comp 1 gasket head, and Sig 25% fuel. Produces low enough idle, smooth transition and adequate full throttle. Good enough for 1/2A RC flying with 5x3 "Rubber Duckie" prop. The results are steady and repeatable which is contrary to what I was getting with an unmodified stock carb. In all, it now works as well as the latter day RC.05 carbs sold by Xenalook and X Model Engines.

Only major downsides to this modification are: New Tarno's are no longer available, carb is heavy, two taps and two drills needed are expensive, and stock carbs are hard to find and are usually quite expensive. Revisited RC.05 carbs. and "Dickybird" throttle sleeve/venturi modded engines work better in my opinion from my own experimentation and are significantly more economical.

Toysareforkids

My Feedback: (4)

I have that same carb & muffler. I have been trying to find where I put them both. Mine are new unused. I did find the peper on it though. I guess I'm getting close to finding it.! I haven'tlooked at it in 20 yrs, I don't even know if they work good! LOL

Pat

Mr Cox

They are strangely made, and not really worth the money have been going for on ebay. The intake diameter is smaller above than below the barrel, hence making the low end richer rather than leaner (which is what needs). It therefore has an extra large airbleed hole, but no adjustment on it.

The carb for the 05RC (available from coxengines.ca) is thereby better as it also has an adjustable airbleed. All the parts are basically available to assemble a Cox 05RC, except for the muffler.

dennis

My Feedback: (90)

The rarest tarno conversion was for the reed valve engines and replaced the tank with a somewhat space hopper arrangement. I had one but a fellow in Sweden really wanted it so it's been gone for a good 15 years.
dennis

astronomer1

Mr Cox: I agree with your comments. They spurred me on to do a little rambling about the theory behind my modified, Tarno Carburetor and it’s adjustable air bleed that does work.

I agree that unmodified “Antique” Tarno Carbs. are probably not worth the money or tinkering time because their stock performance has a lot to be desired. They are quite heavy to boot. Other Carbs. and setups work as well as or better which I mentioned in my original post. Examples include: latter day COX TD05 RC Carbs. and throttle ringed/venturi necked “Dickybirded” TD 049/051 setups work very well. “Dickybird” TD setups work best for me due to their simplicity/reliability, light weight, and good performance. (≈5.5-6K to 19+K RPM)

For me, modifying a Tarno is another story. Tinkering presents challenges that my retired and atrophying engineering brain needs to keep its 65 YO clock wound. Take note, most of what I say concerning the reasons for my enhanced Tarno Carb.’s performance is opinion which is not thoroughly analyzed or provable. My experimentation did however result in a useable carburetor. (I’m thinking about using the previously shown modified Tarno TD.051 engine on an Air Boat rather than a 1/2A Plane because a boat won’t crash if the engine quits). I have a TD 049 that is “Dickybirded” and will work fine for the ACE Whizard I am building. (≈5.5-6K to 19K RPM)

This particular Carburetor running at 5800 ASL after being modified, has smooth transition, fairly high Idle speeds, (≈7K RPM but slow enough for bringing an RC plane down), and lower High-end, (≈18K, again, high enough to fly most 1/2A RC planes). The unmodified carb., ran at (≈ 10K idle to ≈18K high end, with ragged transition).

I don’t know whether the throat as you mentioned, has a larger opening than an exit. I never paid attention or measured it. That would seem to be backward and not according to “Hoyl” or “Murphy”. I do think that when the barrel is closed the pressure around the fuel bar gets lower. This causes more fuel to be sucked resulting in a richer mixture that is out of the ideal combustion range (steicometric** [spell??] ratio). The volume of intake air is also reduced adding to the rich mixture like a choke on an old time car. This results in very poor performance, loading up of the engine, and eventual engine stoppage. This is contrary to what you want to have happen in an RC carburetor. A relatively constant fuel/air **ratio throughout the carb’s. range of operation and a little rich at idle is the ideal in my opinion. A slightly rich idle will facilitate a smoother transition to full throttle. Without being able to control this pressure in a stock Tarno carburetor, performance is significantly compromised. I think the pressure at the fuel nozzle gets lower as the barrel is closed due to higher acceleration of the air in the carb. throat. (Bernoulli’s principal) Try the experiment shown below and do some ancillary thinking while blowing on the spool. Think about acceleration and air pressure and why a venturi or any other airfoil works. Consider why the piece of cardboard doesn’t just blow out of the spool when you blow harder.

To counteract the much richer mixture at idle, and the lowered pressure in the carb. throat, as well as higher suction at the fuel nozzle, the air into the carburetor must be better controlled. A closed Air Bleed creates a richer mixture—Open Air Bleed causes leaner mixture. This is done by providing an air bleed opening which results in increased carb. throat pressure and a leaner mixture due to less fuel being drawn thru the fuel bar. Getting the sweet spot for any given carburetor requires adjustability. “Voila” the adjustable air bleed.